fixup! typo

index.bs

@@ -816,7 +816,7 @@ Objects can be defined to implement a single given interface |A|,
 which means that it also implements all of |A|’s [=inherited interfaces=].
 In addition, an [=includes statement=] can be used
 to define that objects implementing an [=interface=] |A|
-will always also includes the [=interface mixin member|members=]
+will always also include the [=interface mixin member|members=]
 of the [=interface mixins=] |A| [=includes=].
 
 Each interface member can be preceded by a list of [=extended attributes=] (matching